1. Field of the Invention
The invention relates to a light-emitting diode (LED) chip and more particularly to a white-light LED chip and a fabrication method thereof.
2. Description of the Related Art
Light-emitting diode (LED) is a compound semiconductor device that energy transfers electrical current into light. The light emitted by an LED is distributed across a spectrum that is approximately 20˜40 nm wide and has a peak emission wavelength defined by the semiconductor material of a light-emitting layer of the LED. As a consequence of the peak emission wavelength, a single LED cannot emit white light, which is composed of spectral contributions from almost the entire wavelength range of the visible spectrum. FIG. 1 shows a cross section of a conventional white light emitting device 100. The white light emitting device 100 includes a blue-light LED chip 12 disposed in a reflective cup 10 and surrounded by phosphors 14. A glass plate 16 is disposed on the reflective cup 10, covering the LED chip 12 and the phosphors 14. In the exemplary device of FIG. 1, a portion of the blue light emitted by the LED chip 12, and the red and the green light emitted by the phosphor 14 as a result of a partial absorption of the blue light can combined to produce white light.
However, white light generated by sources such as the device illustrated in FIG. 1 is not uniform in color. This non-uniformity is a consequence of the variations in the thickness of the phosphors 14 surrounding the LED chip 12. The variations in the thickness cause spatially non-uniform absorption of blue light and emission of red and green light. Therefore, the generated white light is surrounded by colored rings.
U.S. Pat. No. 6,576,488 discloses a method for conformally coating a light emitting semiconductor structure, such as an LED chip, with a phosphor layer. The method involves electrically coupling an LED chip to a submount, applying a first bias voltage to the submount, and applying a second bias voltage to a solution of charged (pumped) phosphor particles. The electric field created by the two bias voltages induces the phosphor particles to deposit on the conductive surface, such as the surfaces of the submount and the LED chip. However, using the electrophoretic deposition process to form a phosphor layer on a LED chip has some problems. The adhesion between the phosphor particles and the surface of the LED chip is weak, such that the phosphor layer is easily separated from the LED chip. U.S. Pat. No. 6,576,488 discloses using a chemical vapor deposition (CVD) process to form a tinted thin film of a yellow color material on a blue LED element. This method can improve light uniformity and the light output is not heavily dependent on the thickness of a substrate of an LED chip. However, the process of this method is complicated, as passivation layers that cover the LED chip need to be formed. Additionally, it is difficult to control the process for forming the tinted thin film of the yellow color material on the LED chip by the CVD process.
Therefore, a white-light LED chip with uniformity in color that is capable of overcoming the above problems is desired.